Alumina layer with controlled texture

ABSTRACT

A new and refined method to produce α-Al 2 O 3  layers in a temperature range of from about 750 to about 1000° C. with a controlled growth texture and substantially enhanced wear resistance and toughness than the prior art is disclosed. The α-Al 2 O 3  layer of the present invention is formed on a bonding layer of (Ti,Al)(C,O,N) with increasing aluminium content towards the outer surface. Nucleation of α-Al 2 O 3  is obtained through a nucleation step being composed of short pulses and purges consisting of Ti/Al-containing pulses and oxidising pulses. The α-Al 2 O 3  layer according to the present invention has a thickness ranging from about 1 to about 20 μm and is composed of columnar grains. The length/width ratio of the alumina grains is from about 2 to about 12, preferably from about 4 to about 8. The layer is characterized by a strong (104) growth texture, measured using XRD, and by low intensity of (012), (110), (113), (024) and (116) diffraction peaks.

BACKGROUND OF THE INVENTION

The present invention relates to a coated cutting tool insert designedto be used in metal machining. The substrate is a cemented carbide,cermet, ceramics or cBN on which a hard and wear resistant coating isdeposited. The coating exhibits an excellent adhesion to the substratecovering all functional parts thereof. The said coating is composed ofone or more refractory layers of which at least one layer is a stronglytextured alpha-alumina (α-Al₂O₃) deposited in the temperature range offrom about 750 to about 1000° C.

A crucial step in the deposition of different Al₂O₃ polymorphs is thenucleation step. κ-Al₂O₃ can be grown in a controlled way on {111}surfaces of TiN, Ti(C,N) or TiC having the fcc structure. TEM hasconfirmed the growth mode which is that of the close-packed (001) planesof κ-Al₂O₃ on the close-packed {111} planes of the cubic phase with thefollowing epitaxial orientation relationships: (001)_(κ)//(111)_(TiX);[100]_(κ)//[112]_(TiX). An explanation and a model for the CVD growth ofmetastable κ-Al₂O₃ have proposed earlier (Y. Yoursdshahyan, C. Ruberto,M. Halvarsson, V. Langer, S. Ruppi, U. Rolander and B. I. Lundqvist,Theoretical Structure Determination of a Complex Material: κ-Al₂O₃ , J.Am. Ceram. Soc. 82(6)(1999)1365-1380).

When properly nucleated, κ-Al₂O₃ layers can be grown to a considerablethickness (greater than about 10 μm). The growth of even thicker layersof κ-Al₂O₃ can be ensured through re-nucleation on thin layers of, forexample TiN, inserted in the growing κ-Al₂O₃ layer. When nucleation isensured, the κ→α transformation can be avoided during deposition byusing a relatively low deposition temperature (less than about 1000°C.). During metal cutting, the κ→α phase transformation has confirmed tooccur resulting in flaking of the coating. In addition, there areseveral other reasons why α-Al₂O₃ should be preferred in many metalcutting applications. As shown earlier α-Al₂O₃ exhibits better wearproperties in cast iron (U.S. Pat. No. 5,137,774).

However, the stable α-Al₂O₃ phase has been found to be more difficult tobe nucleated and grown at reasonable CVD temperatures than themetastable κ-Al₂O₃. It has been experimentally confirmed that α-Al₂O₃can be nucleated, for example, on Ti₂O₃ surfaces, bonding layers of(Ti,Al)(C,O) or by controlling the oxidation potential using CO/CO₂mixtures as shown in U.S. Pat. No. 5,654,035. The bottom line in allthese approaches is that nucleation must not take place on the111-surfaces of TiC, TiN, Ti(C,N) or Ti(C,O,N), otherwise κ-Al₂O₃ isobtained.

It should also be noted that in the prior-art methods higher depositiontemperatures (about 1000° C.) are usually used to deposit α-Al₂O₃. Whenthe nucleation control is not complete, as is the case in many prior-artproducts, the produced α-Al₂O₃ layers have, at least partly, been formedas a result of the κ-Al₂O₃→α-Al₂O₃ phase transformation. This isespecially the case when thick Al₂O₃ layers are considered. These kindsof α-Al₂O₃ layers are composed of larger grains with transformationcracks. These layers exhibit much lower mechanical strength andductility than the α-Al₂O₃ layers that are composed of nucleatedα-Al₂O₃. Consequently, there is a need to develop techniques to controlthe nucleation step of α-Al₂O₃.

The control of the α-Al₂O₃ polymorph in industrial scale was achieved inthe beginning of the 1990's with commercial products based on U.S. Pat.No. 5,137,774. Later modifications of this patent have been used todeposit α-Al₂O₃ with preferred coating textures. In U.S. Pat. No.5,654,035, an alumina layer textured in the (012) direction and in U.S.Pat. No. 5,980,988 in the (110) direction are disclosed. In U.S. Pat.No. 5,863,640, a preferred growth either along (012), or (104) or (110)is disclosed. U.S. Pat. No. 6,333,103 describes a modified method tocontrol the nucleation and growth of α-Al₂O₃ along the (10(10))direction. US20020155325A1 describes a method to obtain a strong (300)texture in α-Al₂O₃ using a texture modifying agent (ZrCl₄). Theprocesses discussed above use all high deposition temperatures of about1000° C.

US 2004/0028951A1 describes a new state-of-the-art technique to achievea pronounced (012) texture. The commercial success of this kind ofproduct demonstrates the importance to refine the CVD process of α-Al₂O₃towards fully controlled textures.

It is well established that the water gas shift reaction, in the absenceof H₂S or other dopants, is the critical rate-limiting step for Al₂O₃formation, and to a great extent, controls the minimum temperature atwhich Al₂O₃ can be deposited. Further it is well established that thewater-gas shift reaction is very sensitive for deposition pressure.

Extensive work has been done to deposit CVD Al₂O₃ at lower temperatures.Several Al₂O₃ layers using other than AlCl₃—CO₂—H₂ system have beeninvestigated, including AlCl₃—CO—CO₂, AlCl₃—C₂H₅OH, AlCl₃—N₂O—H₂,AlCl₃—NH₃—CO₂, AlCl₃—O₂—H₂O, AlCl₃—O₂—Ar, AlX₃—CO₂ (where X is Cl, Br,I), AlX₃—CO₂—H₂ (where X is Cl, Br, I), AlBr₃—NO—H₂—N₂ andAlBr₃—NO—H₂—N₂. It is emphasised that these studies have been carriedout without dopants (such as H₂S) and the effect of the depositionpressure has not been elucidated.

It is worth noting that none of these systems have been commerciallysuccessful. Consequently, to provide a CVD process for depositing Al₂O₃layers at temperatures below those currently used on a commercial scaleis therefore highly desirable.

U.S. Pat. No. 6,572,991 describes a method to deposit γ-Al₂O₃ at lowdeposition temperatures. This work clearly shows that it is possible toobtain Al₂O₃ layers in the medium temperature range from theAlCl₃—CO₂—H₂ system. However, in this work it was not realised thatnucleation surface controls the phase composition of Al₂O₃ and thatdeposition of α-Al₂O₃ is thus possible at lower deposition temperatures.In the prior-art, it was considered impossible to deposit α-Al₂O₃ at lowtemperatures and it was believed that γ-Al₂O₃ and κ-Al₂O₃ were theunavoidable low temperature phases.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention is to provide a new, improvedalumina layer where the α-Al₂O₃ phase is of nucleated α-Al₂O₃ with astrong, fully controlled (104) growth texture. According to the presentinvention, α-Al₂O₃ with the controlled (104) texture can be obtainedwithin a wide temperature range from about 750 to about 1000° C., whichcan be considered surprising.

In one aspect of the invention, there is provided a cutting tool insertof a substrate at least partially coated with a coating with a totalthickness of from about 10 to about 40 μm, of one or more refractorylayers of which at least one layer is an alumina layer, said aluminalayer being composed of columnar α-Al₂O₃ grains with texturecoefficients

-   -   a) TC(104) greater than about 2.0;    -   b) TC(012), TC(110), TC(113), TC(024) all less than about 1.0;    -   c) TC(116) less than about 1.2;    -   The texture coefficient TC(hkl) is defined as        ${{TC}\left( {{hk}\quad l} \right)} = {\frac{I\left( {{hk}\quad l} \right)}{I_{O}\left( {{hk}\quad l} \right)}\left\{ {\frac{1}{n}{\sum\frac{I\left( {{hk}\quad l} \right)}{I_{O}\left( {{hk}\quad l} \right)}}} \right\}^{- 1}}$    -   where    -   I(hkl)=measured intensity of the (hkl) reflection    -   I_(O)(hkl)=standard intensity according to JCPDS card no 46-1212    -   n=number of reflections used in the calculation (hkl)        reflections used are: (012), (104), (110), (113), (024), (116).

In another aspect of the invention, there is provided a method ofcoating a substrate with an Al₂O₃ layer wherein the α-Al₂O₃ layer iscomposed of columnar α-Al₂O₃ grains with a texture coefficient TC (104)greater than about 2 comprising depositing a (Ti,Al)(C,O,N) bondinglayer on said substrate to provide a nucleation surface for said Al₂O₃,subjecting said nucleation surface to a modification treatment of apulse treatment with a mixture of TiCl₄, AlCl₃ and H₂, a purge with aneutral gas and an oxidizing pulse of a gas mixture including N₂ and CO₂in a ratio of from about 450 to about 650, repeating the modificationtreatment and depositing α-Al₂O₃ having a texture coefficient TC(104)greater than about 2 at a temperature of from about 750 to about 1000°C.

The alumina layer with strong texture outperforms the prior art withrandom or other less developed and incompletely controlled textures.Further, increased toughness can be obtained when deposition is carriedout at lower temperatures. Compared with prior-art products the α-Al₂O₃layer according the present invention is essentially free fromtransformation stresses, consisting of columnar, defect free, α-Al₂O₃grains with low dislocation density and with improved cuttingproperties. The texture-controlled α-Al₂O₃ layers deposited at mediumtemperature (about 800° C.) show enhanced toughness.

DESCRIPTION OF THE DRAWINGS AND FIGURES

FIG. 1 shows a cross-section SEM image (magnification 5000×) of atypical alumina layer according to the present invention deposited on aMTCVD-Ti(C,N) layer. The alumina layer is composed of columnar grains.It is dense with no detectable porosity.

FIG. 2 shows a cross-section SEM image of a typical layer according theprior-art (magnification 6000×) deposited on a MTCVD-Ti(C,N) layer. Thealumina layer is composed of large nearly equiaxed grains. Porosity isvisible in the alumina layer. Interfacial porosity between the aluminalayer and the Ti(C,N) layer is also visible.

DETAILED DESCRIPTION OF THE INVENTION

A method to deposit α-Al₂O₃ with a strong (104) texture in a temperaturerange of from about 750 to about 1000° C. is described. The inventionutilizes short pulses of precursors followed by purging steps with aninert gas such as Ar. After the purge another precursor is applied as ashort pulse. In addition to the texture control, the method can be usedto produce finer grain sizes by increasing the number of nucleationsites.

Al₂O₃ layers according to the present invention outperform the prior-artand are especially suitable be used in toughness demanding stainlesssteel application such as interrupted cutting, turning with coolant andespecially intermittent turning with coolant. The other area is castiron where the edge strength of this kind of alumina layer is superiorto the prior art.

Ti(C,N) is used as an intermediate layer, which can be obtained eitherby conventional CVD or MTCVD, preferably by MTCVD. The present inventionmakes it possible to deposit α-Al₂O₃ at same temperature as is used todeposit the intermediate MTCVD Ti(C,N) layer. Consequently, theheating-up period can be omitted after MTCVD.

To nucleate α-Al₂O₃ with the specified texture, several steps areneeded. First, on the Ti(C,N) layer a bonding layer characterised by thepresence of an Al concentration gradient is deposited. Nitrogen andCH₃CN are applied during deposition of this bonding layer. The aluminiumcontent on the surface of this layer is considerably, about 30%, higherthan in the bonding layer according to U.S. Pat. No. 5,137,774(prior-art) and the bonding layer is obviously containing nitrogen. Thesurface of this bonding layer is subjected to an additionaltreatment(s).

Nucleation is started with a AlCl₃/TiCl₄/H₂ pulse with a duration of 5minutes. After that an Ar purge (duration about 5 minutes) is applied inorder to remove excess Cl⁻ from the surface. After this, an oxidizingpulse is applied using a CO₂/H₂/N₂/Ar (Co₂=about 0.15%, H₂=about 10%,N₂=about 25%, Ar=balance) gas mixture at a pressure of from about 50 toabout 500 mbar, to a temperature of from about 750° to about 1000° C.,depending on the temperature of the subsequent alumina deposition. Inaddition to a relatively low oxidation potential of the gas mixture, theoxidizing step has to relatively short, from about 0.5 to about 5minutes to secure (104) nucleation. These steps should be repeatedseveral times, preferably from about 2 to about 5 times in sequence toincrease the amount of α-Al₂O₃ nuclei. It is noted that if pulsatingnucleation is used, one has to find an optimized combination between theduration of the individual steps and the amount of the steps, otherwisetoo low or excessive oxidization may be obtained. A person skilled inthe art can find the correct procedure by trial and error.

The key to obtain the specified growth texture is the control of theoxidation potential of the CO₂/H₂/N₂/Ar mixture by adjustment of theN₂:CO₂ ratio. This ratio should be from about 450 to about 650,preferably from about 450 to about 550. The use of controlled oxygenpotential in combination with the correct time and number of pulsesenables the correct nucleation mode. Typical pulse times may range fromabout 10 seconds to about 5 minutes depending on the duration of thepulse. The oxidising pulse is again followed by an Ar purge. These stepsshould be repeated several times, preferably from about 2 to about 5times, in sequence to increase the amount of α-Al₂O₃ nuclei. Excessiveoxidisation must be avoided. A person skilled in the art can find thebest and optimised combination between the duration and the amount ofthe steps.

Detailed Description of the Nucleation Steps

-   -   1. Depositing a bonding layer from about 0.1 to about 1 μm thick        in a gas mixture of from about 2 to about 3% TiCl₄, AlCl₃        increasing from about 0.5 to about 5%, from about 3 to about 7%        CO, from about 1 to about 3% CO₂, from about 2 to about 10% N₂        and balance H₂ at from about 750 to about 1000° C., preferably        at about 800° C. and at a pressure of from about 50 to about 200        mbar.    -   2. Purging by Ar for about 5 min.    -   3. Treating the bonding layer in a gas mixture of from about 1        to about 2% TiCl₄ and from about 2 to about 4% AlCl₃ in hydrogen        for about 2 to about 60 min at from about 750 to about 1000° C.,        preferably at about 800° C. and at a pressure of from about 50        to about 200 mbar.    -   4. Purging by Ar for 5 about min.    -   5. Treating in a gas mixture of from about 0.1 to about 0.15%        CO₂ (preferably about 0.15%), from about 10 to about 30% N₂        (preferably from about 22.5 to about 30% when the CO₂ content is        about 15%), about 10% H₂, balance Ar at a pressure of from about        50 to about 200 mbar for about 0.5 to about 5 minutes at a        temperature of from about 750 to about 1000° C., depending on        the temperature for the subsequent deposition of the alumina        layer.    -   6. Purging by Ar for about 5 min.    -   7. Repeating steps 3-6 to obtain the an optimum oxidation level.    -   8. Depositing an alumina layer at a temperature of from about        950 to about 1000° C. and a pressure of from about 50 to about        200 mbar with desired thickness according to known technique or        depositing an alumina layer at from about 750 to about 950 using        higher deposition pressures (from about 200 to about 500 mbar)        together with higher amounts (from about 0.5 to about 1.5%) of        catalysing precursors such as H₂S or SO_(x), preferably H₂S. The        growth of the alumina layer onto the nucleation layer is started        by sequencing the reactant gases in the following order: CO,        AlCl₃, CO₂. The process temperatures of from about 750 to about        1000° C. can be used since the texture is determined by the        nucleation surface.

The present invention also relates to a cutting tool insert of asubstrate at least partially coated with a coating with a totalthickness of from about 15 to about 40 μm, preferably from about 20 toabout 25 μm, of one or more refractory layers of which at least onelayer is an alpha alumina layer. The α-Al₂O₃ layer deposited accordingto the present invention is dense and exhibits a very low defectdensity. It is composed of columnar grains with a strong (104) texture.The columnar grains have a length/width ratio of from about 2 to about12, preferably from about 4 to about 8.

The texture coefficients (TC) for the α-Al₂O₃ according to the presentinvention layer is determined as follows:${{TC}\left( {{hk}\quad l} \right)} = {\frac{I({hkl})}{I_{O}\left( {{hk}\quad l} \right)}\left\{ {\frac{1}{n}{\sum\frac{I\left( {{hk}\quad l} \right)}{I_{O}\left( {{hk}\quad l} \right)}}} \right\}^{- 1}}$

-   -   where    -   I(hkl)=intensity of the (hkl) reflection    -   I_(O)(hkl)=standard intensity according to JCPDS card no 46-1212    -   n=number of reflections used in the calculation (hkl)        reflections used are: (012), (104), (110), (113), (024), (116).

The texture of the alumina layer is defined as follows: TC(104) greaterthan about 2.0, preferably greater than about 3. Simultaneously TC(012),TC(110), TC(113), TC(024) should be all less than about 1.0, preferablyless than about 0.5. Note that the related (012) and (024) reflectionsare also low. However, for this growth mode TC(116) is somewhat higherthan the other background reflections. TC(116) should be less than about1.2, preferably less than about 1.

The substrate comprises a hard material such as cemented carbide,cermets, ceramics, high speed steel or a super hard material such ascubic boron nitride (CBN) or diamond, preferably cemented carbide orCBN. With CBN is herein meant a cutting tool material containing atleast about 40 vol-% CBN. In a preferred embodiment, the substrate is acemented carbide with a binder phase enriched surface zone.

The coating comprises a first layer adjacent the body of CVD Ti(C,N),CVD TiN, CVD TiC, MTCVD Ti(C,N), MTCVD Zr(C,N), MTCVD Ti(B,C,N), CVD HfNor combinations thereof preferably of Ti(C,N) having a thickness of fromabout 1 to about 20 μm, preferably from about 1 to about 10 μm, and saidα-Al₂O₃ layer adjacent said first layer having a thickness of from about1 to 40 μm, preferably from about 1 to about 20 μm, most preferably fromabout 1 to about 10 μm. Preferably, there is an intermediate layer ofTiN between the substrate and said first layer with a thickness of lessthan about 3 μm, preferably from about 0.5 to about 2 μm.

In one embodiment, the α-Al₂O₃ layer is the uppermost layer.

In another embodiment, there is a layer of carbide, nitride,carbonitride or carboxynitride of one or more of Ti, Zr and Hf, having athickness of from about 0.5 to about 3 μm, preferably from about 0.5 toabout 1.5 μm, atop the α-Al₂O₃ layer. Alternatively this layer has athickness of from about 1 to about 20 μm, preferably from about 2 toabout 8 μm.

In yet another embodiment, the coating includes a layer of κ-Al₂O₃and/or γ-Al₂O₃ preferably atop the α-Al₂O₃. with a thickness of from 0.5to 10, preferably from 1 to 5 μm.

The invention is additionally illustrated in connection with thefollowing examples, which are to be considered as illustrative of thepresent invention. It should be understood, however, that the inventionis not limited to the specific details of the examples.

EXAMPLE 1

Cemented carbide cutting inserts with a composition of 5.9% Co andbalance WC (hardness about 1600 HV) were coated with a layer of MTCVDTi(C,N). The thickness of the MTCVD layer was about 2 μm. On to thislayer two different layers consisting of about 10 μm α-Al₂O₃ weredeposited:

Layer a) contained a (104) textured layer and was deposited according tothe present invention at 1000° C. The detailed process data is given inTable 1.

Layer b) was deposited according to the prior art.

Layer c) contained a (104) textured layer and was deposited according tothe present invention at 800° C. The detailed process data is given inTable 2. TABLE 1 Deposition process for a Layer a) with (104) texture at1000° C.: Step 1: Bonding layer Gas mixture TiCl₄ = 2.8% AlCl₃ =0.8-4.2% CO = 5.8% CO₂ = 2.2% N₂ = 5-6% Balance: H₂ Duration 60 minTemperature 1000° C. Pressure 100 mbar Step 2: Purge Gas Ar = 100%Duration 5 min Temperature 1000 C. Pressure 50 mbar Step 3: Pulse 1 Gasmixture TiCl₄ = 1.6% AlCl₃ = 2.8 H₂ = Balance Duration 2-5 min dependingon the amount of pulses. Temperature 1000 C. Pressure 50 mbar Step 4:Purge Gas Ar = 100% Duration 5 min Temperature 1000 C. Pressure 50 mbarStep 5: Pulse 2 Gas mixture CO₂ = 0.05% N₂ = 25% Balance: H₂ Duration0.5-1 min depending on the amount of pulses. Temperature 1000° C.Pressure 100 mbar Step 6: Purge Gas Ar = 100% Duration 5 min Temperature1000 C. Pressure 50 mbar Step 7: Nucleation step Gas mixture AlCl₃ =3.2% HCl = 2.0% CO₂ = 1.9% Balance H₂ Duration 60 min Temperature 1000°C. Pressure 210 mbar Step 8: Deposition Gas mixture AlCl₃ = 4.2% HCl . .. = 1.0% CO₂ = 2.1% H₂S = 0.2% Balance: H₂ Duration 520 min Temperature1000° C. Pressure 50 mbar

It should be noted that the steps 3-7 can be repeated 5-10 times insequence in order to obtain grain refinement and the strong desiredtexture. The amount of pulses can be even higher if the duration of step6 is reduced. In this example the steps 3-7 were repeated 3 times withdurations of 0.6 minutes. TABLE 2 Deposition process for a Layer c) with(104) texture at 780° C.: Step 1: Bonding layer Gas mixture TiCl₄ = 2.8%CH₃CN = 0.7% AlCl₃ = increasing from 0.8 to 4.2% CO = 5.8% CO₂ = 2.2% N₂= 5% Balance: H₂ Duration 40 min Temperature 780° C. Pressure 100 mbarStep 2: Purge Gas Ar = 100% Duration 5 min Temperature 780° C. Pressure50 mbar Step 3: Pulse 1 Gas mixture TiCl₄ = 1.6% AlCl₃ = 2.8 H₂ =Balance Duration 5 min. Temperature 780° C. Pressure 50 mbar Step 4:Purge Gas Ar = 100% Duration 5 min Temperature 780° C. Pressure 50 mbarStep 5: Pulse 2 Gas mixture CO₂ = 0.05% N₂ = 25% H₂ = 10% Balance: ArDuration 2 min Temperature 780° C. Pressure 100 mbar Step 6: Purge GasAr = 100% Duration 5 min Temperature 780° C. Pressure 50 mbar Step 7:Nucleation step Gas mixture AlCl₃ = 3.2% HCl = 2.0% CO₂ = 1.9% BalanceH₂ Duration 60 min Temperature 780° C. Pressure 50 mbar Step 8:Deposition Gas mixture AlCl₃ = 4.1% HCl = 1.0% CO₂ = 2.3% H₂S = 0.9%Balance: H₂ Duration 600 min Temperature 780° C. Pressure 350 mbarSteps 3-6 were repeated three times.

EXAMPLE 2

Layers a, b and c were studied using X-ray diffraction. The texturecoefficients were determined are presented in Table 3. As clear fromTable 3 the layer according to the present invention exhibits a strong(104) texture when deposited either at 1000° C. or 780° C. Typically,for this growth mode the (116) reflection is somewhat more profound thanthe other background reflections. TABLE 3 Hkl Invention, layer a Priorart, layer b, Invention, layer c 012 0.24 0.97 0.49 104 4.30 1.14 3.13110 0.06 0.95 0.49 113 0.19 0.99 0.41 024 0.27 0.86 0.49 116 0.94 1.090.99

EXAMPLE 3

Layers a) and b) were studied using Scanning Electron Microscopy. Thecross section images of the layers are shown in FIGS. 1 and 2,respectively. The differences in microstructure and morphology areclear.

EXAMPLE 4

The layers a) and b) from the Example 1 were tested with respect to edgechipping in longitudinal turning of cast iron.

-   -   Work piece: Cylindrical bar    -   Material: SS0130    -   Insert type: SNUN    -   Cutting speed: 400 m/min    -   Feed: 0.4 mm/rev    -   Depth of cut: 2.5 mm    -   Remarks: dry turning

The inserts were inspected after 2 and 4 minutes of cutting. As clearfrom Table 4 the edge toughness of the prior art product wasconsiderably enhanced when the layer was produced according to thepresent invention. TABLE 4 Flaking of the edge line Flaking of the edgeline (%) after 2 minutes (%) After 6 minutes Layer a (Invention) 0 6Layer b 12 22

EXAMPLE 5

The layer produced according to the present invention was compared witha market leader, referred here as Competitor X. This coating is composedof MTCVD Ti(C,N) and α-Al₂O₃. XRD was used to determine the texturecoefficients for these competitor coatings. Two inserts from CompetitorX were randomly chosen for XRD. Table 5 shows the obtained TCs for theCompetitor X. The coatings from Competitor X exhibit a random textureand can be compared with the present invention, Table 1. TABLE 5 HklTC(hkl) 012 0.71 0.57 104 0.92 0.86 110 1.69 1.92 113 0.48 0.40 024 1.161.14 116 1.04 1.11The X-rayed inserts from the competitor X were compared with insertsproduced according to the present invention, Layer a).

Two inserts produced according to the present invention were comparedwith the two Competitor X inserts with respect to flank wear resistancein face turning of ball bearing material: Work piece: Cylindrical tubes(Ball bearings) Material: SS2258 Insert type: WNMG080416 Cutting speed:500 m/min Feed: 0.5 mm/rev Depth of cut: 1.0 mm Remarks: Dry turning

Tool life criterion: Flank wear>0.3 mm, three edges of each variant weretested. Results: Tool life (min) Layer a 25.0 (invention) Layer a 23.5(invention) Competitor 1 14.5 (prior art) Competitor 2 15.5 (prior art)

EXAMPLE 6

Layer a), b) and c) deposited on Co-enriched substrates were tested withrespect to toughness in longitudinal turning with interrupted cuts. Workpiece: Cylindrical slotted bar Material: SS1672 Insert type:CNMG120408-M3 Cutting speed: 140 m/min Feed: 0.1, 0.125, 0.16, 0.20,0.25, 0.315, 0.4, 0.5, 0.63, 0.8 mm/rev gradually increased after 10 mmlength of cut Depth of cut: 2.5 mm Remarks: dry turning

Tool life criteria: Gradually increased feed until edge breakage. 10edges of each variant were tested. TABLE 6 Mean feed at breakage(mm/rev) Layer a (invention) 0.24 Layer b (prior art) 0.12 Layer c(invention) 0.32

The test results show (Table 6) that layers according to the presentinvention exhibited clearly better toughness behaviour than theprior-art (layer b).

EXAMPLE 7

Cubic boron nitride (CBN) insert containing about 90% of polycrystallineCBN (PCBN) were coated according to the present invention and accordingto prior art layer discussed in Example 1. The coated CBN was comparedwith uncoated CBN insert in cutting of steel containing ferrite. It isknown that B has a high affinity to ferrite and diffusion wear occurs athigh cutting speeds. As shown in Table 7 the layer according to thepresent invention is superior to the prior art. Work piece: Cylindricalbar Material: SS0130 Insert type: SNUN Cutting speed: 750 m/min Feed:0.4 mm/rev Depth of cut: 2.5 mm Remarks: dry turning

TABLE 7 Life time (min) Coated CBN (Invention) 26 Coated according toprior art 11 Uncoated CBN 9

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without department from thespirit and scope of the invention as defined in the appended claims.

1. Cutting tool insert of a substrate at least partially coated with acoating with a total thickness of from about 10 to about 40 μm of one ormore refractory layers of which at least one layer is an alumina layer,said alumina layer being composed of columnar α-Al₂O₃ grains withtexture coefficients a) TC(104) greater than about 2.0; b) TC(012),TC(110), TC(113), TC(024) all less than about 1.0; c) TC(116) less thanabout 1.2; The texture coefficient TC(hkl) is defined as${{TC}\left( {{hk}\quad l} \right)} = {\frac{I\left( {{hk}\quad l} \right)}{I_{O}\left( {{hk}\quad l} \right)}\left\{ {\frac{1}{n}{\sum\frac{I\left( {{hk}\quad l} \right)}{I_{O}\left( {{hk}\quad l} \right)}}} \right\}^{- 1}}$where I(hkl)=measured intensity of the (hkl) reflectionI_(O)(hkl)=standard intensity according to JCPDS card no 46-1212n=number of reflections used in the calculation (hkl) reflections usedare: (012), (104), (110), (113), (024), (116).
 2. The cutting toolinsert of claim 1 wherein said alumina layer is composed of columnargrains with a length/width ratio from about 2 to about
 12. 3. Thecutting tool insert of claim 1 wherein said substrate comprises cementedcarbide, CBN or sintered CBN alloy.
 4. The cutting tool insert of claim1 wherein the coating comprises a first layer adjacent the body of CVDTi(C,N), CVD TiN, CVD TiC, MTCVD Ti(C,N), MTCVD Zr(C,N), MTCVDTi(B,C.N), CVD HfN or combinations thereof having a thickness of fromabout 1 to about 20 μm and said α-Al₂O₃ layer adjacent said first layerhas a thickness of from about 1 to about 40 μm.
 5. The cutting toolinsert of claim 1 wherein the α-Al₂O₃ layer is the uppermost layer. 6.The cutting tool insert of claim 1 wherein a layer of carbide, nitride,carbonitride or carboxynitride of one or more of Ti, Zr and Hf, having athickness of from about 0.5 to about 12 μm is atop the α-Al₂O₃ layer. 7.The cutting tool insert of claim 1 wherein a layer of κ-Al₂O₃ or γ-Al₂O₃is atop the α-Al₂O₃ with a thickness of from about 0.5 to about
 10. 8.The cutting tool insert of claim 1 wherein there is a layer of TiNbetween the substrate and said first layer with a thickness of less thanabout 3 μm.
 9. The cutting tool insert of claim 1 wherein said substratecomprises a cemented carbide with a binder phase enriched surface zone.10. The cutting tool insert of claim 1 wherein the coating has athickness of from about 15 to about 25 μm
 11. The cutting tool insert ofclaim 10 wherein the α-Al₂O₃ grains have texture coefficients, a) TC(104) greater than about 3, b) TC (012), TC (110), TC (113), TC (024)all less than about 0.5 and c) TC (116) less than about 1.0.
 12. Thecutting tool of claim 2 wherein the columnar grains have a length/widthratio of from about 4 to about
 8. 13. The cutting tool insert of claim 4wherein the said first layer has a thickness of from about 1 to about 10μm and said α-Al₂O₃ layer has a thickness of from about 1 to about 20μm.
 14. The cutting tool insert of claim 13 wherein in said α-Al₂O₃layer has a thickness of from about 1 to about 10 μm.
 15. The cuttingtool insert of claim 6 wherein the said layer of carbide, nitride,carbonitride or carboxyitride has a thickness of from about 0.5 to about6 μm.
 16. The cutting tool insert of claim 7 wherein the said layer ofκ-Al₂O₃ or γ-Al₂O₃ has a thickness of from about 1 to about 5 μm. 17.The cutting tool insert of claim 8 wherein the TiN layer has a thicknessof from about 0.5 to about 2 μm.
 18. A method of coating a substratewith an Al₂O₃ layer wherein the α-Al₂O₃ layer is composed of columnarα-Al₂O₃ grains with a texture coefficient TC (104) greater than about 2comprising depositing a (Ti,Al)(C,O,N) bonding layer on said substrateto provide a nucleation surface for said Al₂O₃, subjecting saidnucleation surface to a modification treatment of a pulse treatment witha mixture of TiCl₄, AlCl₃ and H₂, a purge with a neutral gas and anoxidizing pulse of a gas mixture including N₂ and CO₂ in a ratio of fromabout 450 to about 650, repeating the modification treatment anddepositing α-Al₂O₃ having a texture coefficient TC(104) greater thanabout 2 at a temperature of from about 750 to about 1000° C.
 19. Themethod of claim 18 wherein each said oxidizing pulse treatment isconducted for a time of from about 0.5 to about 5 minutes.
 20. Themethod of claim 18 wherein the neutral gas is argon.
 21. The method ofclaim 18 wherein the mixture of TiCl₄, AlCl₃ and H₂ comprises a mixtureof about 1 to about 3% TiCl₄ from about 2 to about 4% AlCl₃, balance H₂.22. The mixture of claim 21 wherein the said pulse treatment with TiCl₄,Al₂O₃ and H₂ is conducted for a time of from about 2 to about 60 minutesat a temperature of from about 750 to 1000° C. and a pressure of fromabout 50 to about 200 mbar.
 23. The method of claim 18 wherein theoxidizing pulse comprises a mixture of from about 0.05 to about 0.1% CO₂from about 20 to about 65% N₂, about 10% H₂, balance Ar.
 24. The methodof claim 23 wherein the oxidizing pulse is conducted for a time of fromabout 0.5 to about 5 minutes, a temperature of from about 750° to about1000° C. and a pressure of from about 50 to about 500 mbar.
 25. Themethod of claim 18 wherein the α-Al₂O₃ deposition is conducted at atemperature of from about 950 to about 1000° C.
 26. The method of claim18 wherein the α-Al₂O₃ deposition is conducted at a temperature of fromabout 750 to 950° C. at a pressure of from about 200 to about 500 mbar.27. The method of claim 26 wherein there is present a catalyzingprecursor in an amount of from about 0.5 to about 1.5%.
 28. The methodof claim 27 wherein the catalyzing precursor is H₂S or SO_(x).
 29. Themethod of claim 28 wherein the catalyzing precursor is H₂S.